Transmission of information by the use of light over optical fibers is widely used in long-haul telecommunication systems. Optical signals are generated, transported along optical fibers and detected to regenerate the original electronic signal with as little change as possible. Fibers are substituted for other transmission media and all signal processing is done electronically, resulting in lowered cost and high quality digital transmission.
As fiber optic applications technology develops direct optical processing of signals without conversion to electronic signals will be required. Optical fiber systems will be applied in computer networks, for example, in multiple access computer networks. Such applications will require optical fiber devices such as amplifiers, multiplex/demultiplexes, splitters, couplers, filters, equalizers, switches and other optical signal processors.
An economical low-loss, easily and reproducibly manufactured single-mode optical fiber filter, the design of which can be adapted to a desired bandwidth, FSR and finesse is an important component for such fiber optic systems. A fiber Fabry-Perot (FFP) interferometric filter is such a component.
The Fabry-Perot (FP) Interferometer was first described by C. Fabry and A. Perot in 1897 (Ann. Chem. Phys., 12:459-501) and has since found wide use in a variety of applications of optical filters. The basic structure and operation of the FP interferometer is well-known in the art and is described in many physics and optics texts (see, for example, E. Hecht "Optics" 2nd. Edition (1987) Addison-Wesley, Reading, Mass., p. 369). This interferometer consists of an optical cavity formed between two typically highly reflecting, low-loss, partially transmitting mirrors. Lenses are typically used to collimate divergent optical beams for processing through the FP interferometer.
While single-mode optical fibers can be used with lensed conventional FP interferometers, lenses with large beam expansion ratios are required and result in reduced stability and poor optical performance. The adaptation of FP cavities for optical fiber filters had been hindered by the lack of practical designs for FFPs with appropriate optical properties. Recently, FFPs which possess optical properties suitable for telecommunication applications have been described. These FFPs consist of two highly reflective, preferably plane-parallel mirrors, forming the optical cavity through at least a portion of which, in most cases, a length of single-mode optical fiber extends. This basic design eliminates the need for collimating and focusing lenses, improves stability and optical performance and makes the FFPs compatible with single-mode optical fibers and other fiber devices.
The transmission characteristics of a typical FFP of length, l.sub.c have been described previously, for example see U.S. Pat. No. 5,289,552. The fractional transmitted power, I.sub.t /I.sub.in, through the optical cavity is ##EQU1## where l.sub.c is the cavity length which is the length of single-mode optical fiber and any fiber gaps or spacing between the mirrors which form the cavity, n is the index of refraction of the cavity material, F is the finesse of the cavity and k is the insertional loss. For the loss-less case, k is 1 and the ideal F is dependent only on the reflectivities of the mirrors (R, where the reflectivities of the mirrors are assumed to be equal) and ##EQU2## For a fixed value of n, when nl.sub.c =m.lambda./2, where m is an integer, equation 1 has maxima corresponding to a resonance condition within the cavity. Incident light with .lambda. that is an integer multiple of the cavity optical path length (nl.sub.c) is transmitted with little attenuation. Incident light of other wavelengths is highly attenuated. For a given m, changing l.sub.c or .lambda. results in a shift of all transmission maxima. Insertion loss is the minimum loss through the FFP and is equal to -10 log k or -10 log (I.sub.1 /I.sub.in). The difference between the frequencies of the resonance peaks, for constant l.sub.c and .lambda., is the free spectral range (FSR)=c/2nl.sub.c, where c=3.times.10.sup.8 m/s. An FFP is tuned between successive resonance maxima by, for example, changing l.sub.c. (Alternatively, tuning of the FFP can be accomplished by changing n.) The bandwidth (BW) is the full width at half maximum. The finesse of the filter, F=FSR/BW, can be measured experimentally by measuring the ratio of FSR to BW from the transmission curves generated by varying l.sub.c with constant .lambda.. Measuring F in this manner accounts for all non-dispersive losses including mirror absorption, diffraction and alignment losses. If .lambda. is varied to generate transmission curves, dispersive properties of the mirrors, fibers, and cavity modes are also included in the measured FSR.
In 1987, J. Stone and L. W. Stulz described three configurations of FFP interferometric filters (Elect. Lett., 23(15):781-783, 1987) that span a wide spectrum of bandwidths and tuning ranges. The Type I FFP is a long cavity FFP in which mirrors are deposited at the ends of a continuous fiber. In this FFP, the fiber can be stretched by piezoelectric transducers (PZTs) to produce tuning of the bandwidth (BW) over the free spectral range (FSR).
The Type II FFP of Stone and Stulz is a gap resonator which has no optical fiber inside the optical cavity and so can exhibit significant losses. Due to such losses, the useful cavity length of this type of FFP is less than about 5 .mu.m.
The Type III FFP is better suited to telecommunication applications than either of the other types of FFPs. It has an internal waveguide interposed between external fiber ends. Mirrors are positioned at an external fiber end and at one end of the waveguide. The waveguide is comprised within the optical cavity. The optical cavity also contains a fiber gap, for example between the waveguide and one of the external fiber ends, the width of which is fixed or can be changed to tune the filter.
The ferrule components and waveguide of Type II and III FFPs must be axially aligned to high precision in order to minimize transmission loss. Type II and III FFPs are the subject of U.S. Pat. No. 4,861,136. This patent relates to FFPs which are tuned by use of PZTs to change the cavity length. In order to use PZTs to change resonance cavity length without detriment to alignment, elaborate alignment brackets and fixtures are necessary.
U.S. Pat. No. 5,062,684 describes an improved tunable FFP filter in which the resonance cavity is formed by two wafered ferrules with mirrors embedded between the wafer and the ferrule and axially disposed optical fibers. The two ferrules are positioned in the filter configuration with mirrors opposed and the optical fibers of the ferrules aligned. The resonance cavity formed between the embedded mirrors contains a fiber gap between the wafered ends of the ferrules. The ferrule combination is held in alignment by an alignment fixture including piezoelectric transducers which function to change the resonance cavity length on application of a voltage to the transducer. A support fixture useful for holding a FFP ferrule assembly in axial alignment is described in EP patent application 0 457 484. This fixture also provides PZTs for changing the cavity length and means for minor adjustment of alignment.
A major problem of FFP filters is signal loss due to wavelength drift as a function of the change in cavity length of the filter with temperature and insertional loss. An uncompensated FFP, like that of U.S. Pat. No. 5,062,684 or EP application 457,484, can exhibit a relatively large change in cavity length, of the order 0.05 .mu.m/.degree.C. This can represent a drift of a full FSR (free spectral range) over 15.degree. C. See C. M. Miller and F. J. Janniello (1990) Electronics Letters 26:2122-2123.
Control circuitry has been employed with PZT-tuned FFPs to lock the filter onto a wavelength over a wide temperature range (I. P. Kaminow (1987) Electronics Letters 23:1102-1103 and D. A. Fishman et al. (1990) Photonics Technology Letters pp.662-664). In these systems, control voltage swings of several tens of volts were required to compensate for the relatively large change in cavity length with temperature. Wavelength locking of FFP filters can prevent signal loss, however since approximately 20 volts applied to a PZT is needed to tune through an FSR, a total power supply range of about 60 volts is needed to maintain the wavelength lock over an operationally useful temperature range of about 30.degree. C. (Fishman et al. supra).
Miller and Janniello (1990) supra described passive temperature compensation of PZT-tuned FFPs. Since PZTs require a higher voltage at higher temperature to maintain a given length, cavity length effectively decreases with increasing temperature (with constant voltage). Thus, the PZT-tuned FFP has a negative temperature coefficient. Addition of a material having a positive temperature coefficient in series with the PZTs, for example aluminum blocks, was found to compensate for the negative temperature coefficient of the PZTs. This method of passive compensation significantly reduced the voltage requirements for FFP locking circuits such that .+-.12 volt power supplies, such as are conventionally employed in computer systems, could be employed for locking.
The production yield of highly accurate, passively compensated FFPs has been low. This is due in part to the fact that it is difficult to obtain the required degree of passive temperature compensation in any particular filter. The filter assembly must be entirely constructed before the extent of compensation achieved can be tested. With such passive compensation there is no means for adjusting the temperature coefficient of the filter after the filter has been fabricated. FFPs are often over or under compensated.
The present invention provides FFPs which overcome the difficulties described above. In the filters of this invention, the temperature coefficient can be selectively varied after fabrication of the filter in order to minimize the variation of the cavity length as a function of temperature. The filter holders and methods described herein provide FFPs which on average have a significantly lower temperature coefficient than previously described temperature compensated FFPs.
The FFPs of this invention can be miniaturized for use in applications in which device size is a concern. FFPs having dimensions of less than about 6.8 mm in height, 10 mm in width and 30 mm in length, in particular, miniaturized FFPs of this invention can be used in circuit broad applications. Circuit board spacing limits the height of circuit board components to about 6.8 mm. Miniaturized FFPs of this invention retain excellent thermal properties. Miniaturized FFPs which display wavelength drift less than 1 FSR/100.degree. C. (-25.degree. C. to 75.degree. C.) and less than 1 dB insertion loss over the same temperature range have been constructed.
Application of the FFP designs and methods herein for filter fabrication and the use of the ferrule holders herein result in significantly improved manufacturing yield of FFPs having desirable optical characteristics and low thermal coefficients.